Joining of component parts together with a fused glass seal to form a composite article is a well-cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics.
In making a fusion-type seal, the material must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adherent, hermetic bond. For many purposes, it is desirable to maintain the sealing temperature as low as possible. This is particularly true in electrical and electronic articles where thermally sensitive parts or coatings are commonly employed.
Accordingly, considerable attention has been given to lead glasses as low temperature sealing glasses. For example, stable sealing glasses, having softening points in the 430.degree.-500.degree. C. range and coefficients of thermal expansion in the 70-90.times.10.sup.-7 /.degree.C. range, are disclosed in U.S. Pat. No. 2,642,633 (Dalton). Subsequent studies centered on lead-zinc borate type glasses that were subject to thermal devitrification or crystallization. These glasses were intensively investigated in the search for cathode ray tube sealing materials.
For many sealing and coating purposes, glasses are used in powder form, termed glass frits. One such application is forming a seal between the funnel and panel members of a cathode ray tube. Sealing glass frits are commonly mixed with an organic vehicle, such as amyl acetate, to form a flowable or extrudable paste. This mixture may then be applied to a sealing surface, e.g., the peripheral rim of a funnel or panel member. It is also known to incorporate mill additions in a glass frit mixture, a primary reason being modification and/or control of the ultimate coefficient of thermal expansion in a seal.
Copending application Ser. No. 924,107, filed Aug. 3, 1992 in the names of Aitken et al., and assigned to the same assignee as the present application, discloses non-lead sealing glasses especially designed for use in cathode ray tube (CRT) sealing frits. These glasses are SnO-ZnO-P.sub.2 O.sub.5 glasses that have compositions that, as calculated on a mole percent basis, consist essentially of 25-50% P.sub.2 O.sub.5 plus SnO and ZnO in amounts such that the mole ratio of SnO:Zno is in the range of 1:1 to 5:1.
The glasses there disclosed have proven quite satisfactory for CRT sealing purposes. However, for electrical and electronic sealing purposes, it would be highly desirable to have sealing glasses with even lower sealing temperatures. This means that the desired glasses would have good flow characteristics at such lower sealing temperatures.
A major consideration in a sealing glass frit is that it provides low residual strain in a seal. This generally requires that the ultimate seal have a coefficient of thermal expansion (CTE) that is a reasonably close match for the CTEs of the parts being sealed.
Electronic and electrical sealing applications tend to involve a large number of different materials. Further, these materials tend to have CTEs that vary over a wide range. The required CTEs are generally lower than the values of 95-105.times.10.sup.-7 /.degree.C. encountered in sealing CRT components. Thus, typical requirements are a CTE of 65-70 to match alumina; 30-40 to match silicon and borosilicate glasses; and 0-10 to match certain glass-ceramics and fused silicas (all values.times.10.sup.-7 /.degree.C.).
In addition to flow and expansion compatibility, a sealing glass frit can desirably possess a number of other favorable characteristics. These include good wettability of the glass parts being sealed, solubility in a common industrial solvent for salvage purposes, and compatibility with organic vehicles. In particular, a frit can desirably be compatible with amyl acetate, a vehicle currently used commercially with lead glass frits.
Lead-zinc borate sealing glasses, both crystallizing and non-crystallizing, have been used over a long period of time, and have proven very successful. Nevertheless, there has been a continuing desire for a sealing glass frit having all the favorable characteristics of a lead-zinc borate glass, but with a somewhat lower sealing temperature. Furthermore, as a health and safety measure, strenuous efforts are being made to avoid use of lead compounds as far as possible.
Accordingly, a basic purpose of the present invention is to provide a lead-free sealing glass frit. A further purpose is to provide a lead-free glass frit having characteristics particularly suited to forming a fusion seal between components for electronic and electrical devices. Another purpose is to provide a means of altering the effective CTEs of the lead-free glasses to extend their potential utility to sealing components having lower CTEs. Still another purpose is to provide a mid-temperature sealing glass frit capable of forming a fusion seal at a temperature in the range of 400-430.degree. C. Yet another purpose is to increase the strength of seals prepared from the frits by the inclusion of reinforcing fibers and fillers therein.